DEFECTS IN BOILER EVAPORATOR TUBES - A DETAILED CASE-STUDY CONCERNINGDEFECT INITIATION AND EXTENSION PROCESSES

The present report attempts to describe a detailed failure analysis th at was conducted on a series of full and partial through-thickness tub e wall defects. Essentially two different defects were observed. One w as a full through-thickness defect, the shape of which strongly sugges ts that it was driven by an active corrosion-dominated process. The se cond type of defect was partial through-thickness in nature and exhibi ted a common development route inasmuch as it extended through the eva porator tube wall by at least four discrete events or defect extension processes, viz. (i) semi-elliptical pit formation, approximately 0.3 mm deep; (ii) thick porous-like magnetite growth (up to 400 mum thick) at the base of the pit; (iii) heavy or active corrosion tunnelling, a pproximately 1.3 mm in diameter; and (iv) an environmentally assisted crack (EAC) fatigue crack propagation region which varied between 0.5 and 1.4 mm in size. In some instances, a fifth defect extension event, which was corrosion dominated, was observed at the end of the fatigue -dominated event (iv). This event was similar to event (iii). Detailed fractography demonstrated that pit formation was the result of a mixe d transgranular and intergranular corrosion process while the corrosio n tunnelling was caused by a predominantly intergranular corrosion act ivity. The fatigue-dominated ( event (iv)) fracture surface region con tained a mixture of flat, cleavage-like facets which are indicative of an EAC growth process, and a fissured, ductile striated, failure mode which is commonplace in the case of pure, mechanically driven, fatigu e processes. In the case of the full through-thickness defect, only an intense intergranular corrosion process was evidenced. The likelihood of these discrete events being caused by on-load or off-load process is discussed, while the presence of localised concentrations of sulphu r and chlorine were detected on the corrosion-tunnelled and EAC fatigu e growth surface regions.